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From the tuning world that I have come from, reducing the restriction in an exhaust system and allowing the engine to breath better always requires additional tuning to compensate for this .
I am not sure about how the BMW M6 ecu handles this, it may have parameters to compensate within a certain range , but looking at you dyno graph it seems that the ecu is not running O2 feedback . This is were it self tunes itself if the values of the AFR exceeds a certain range. This is a great idea but relies on one aspect , the lambda sensor is accurate. A lot of GM motor cars have this feature as they have 2 banks of sensors.
Then again , seeing the AFR reaching such a high value and the ecu reducing the power , says one thing about these cars , the ecu is pretty good . I bet it is also reducing boost . It's a shame you haven't got boost reading on the dyno graph aswell.
On the other hand , it could be a faulty wideband sensor on the dyno , giving you wrong readings . Get it dyno'd again and but put the piggy back to stock setting then slowly increase the boost on each of the run . Make sure they log the boost , AFR, torque ,hp on the same graph . But in the mean time , knock the boost to stock to be safe .
I am hoping to stick mine on the dyno this weekend to see what the AFR is like on boost, don't get to involved in chasing numbers on a dyno, tuning is all about power delivery and drive ability, pointless having all the power up top when there is nothing mid range .
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